BP finished pumping cement into its ruptured oil well in the Gulf of Mexico on Thursday to seal off the source of the world's worst offshore spill, paving the way to permanently plug the blow-out later this month.

The daylong cementing operation followed earlier injections of heavy drilling mud this week that had subdued the upward pressure of oil and gas inside the deep-sea Macondo well. The crippled wellhead was provisionally capped in mid-July.

"This is not the end, but it will virtually assure us that no oil will be leaking into the environment," retired Coast Guard Admiral Thad Allen, who oversees the U.S. oil spill response operation, said at a briefing in Washington.

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"Monitoring of the well is under way in order to confirm the effectiveness of the procedure," BP said in a statement announcing completion of the cementing work.

The so-called "static kill" at the top of the well is due to be finished off with a "bottom kill" later in August with more mud and cement injected through a relief bore being drilled into the ruptured well shaft. This relief well is regarded as the final step in plugging the reservoir 13,000 feet beneath the seabed.

"I will declare this well dead once we've intercepted the annulus (the space between the well pipe and surrounding rock) and we've assessed how much mud or cement we need to do from the bottom to finally kill this well," Allen said.

Allen said BP would likely resume drilling the relief well 24 to 36 hours after the cementing was done, with the initial intercept expected within five to seven days after that.

Progress in shutting off the cause of an environmental disaster for the U.S. Gulf Coast came as a relief for both BP, whose image and stock took a beating, and U.S. President Barack Obama, whose approval ratings suffered over criticism of his administration's handling of the spill.

Photo shows a remotely operated robotic arm at work near the containment
capping stack, in this image captured from a BP live video feed from
the Gulf of Mexico August 3, 2010. Credit: REUTERS/BP/Handout